FIG. 3 of the accompanying drawings shows a cast iron camshaft A, which is manufactured as follows: First, raw materials are melted into a molten metal. The raw materials include scrap steel, return scrap such as scrap from guides such as sprues and risers, defective cast iron members, and pig iron.
The molten metal is poured into a mold such as a sand mold, a metal mold, or the like, and then cooled and solidified into a black or formed product which is of a shape substantially corresponding to the final product, i.e., the cast iron camshaft A. The molten metal is cooled at a cooling rate which is generally about 240° C./minute for sand molds and about 1000° C./minute for metal molds.
Therefore, using metal molds is effective to greatly increase the cooling rate for the molten metal. Stated otherwise, using metal molds is advantageous in that blanks or formed products and camshafts A or final products can efficiently be manufactured.
When a molten metal is cooled and solidified at a high cooling rate, a chilled structure of cementite (Fe3c) is formed in the surface layer of the produced casting. Since the chilled structure is very hard, the wear resistance of the surface layer of the casting is excessively high. If the casting is shaped into the camshaft A, then it is necessary to cut and otherwise machine the surface layer of the casting to predetermined dimensions. However, the excessively high resistance of the surface layer of the casting makes it difficult to cut and otherwise machine the surface layer of the casting.
To make the casting easily machinable, the casting is heated in a non-oxidizing atmosphere such as a reducing atmosphere or an inactive atmosphere to decompose the chilled structure into austenite (γ-Fe) and graphite, thus eliminating the chilled structure.
Finally, the heat-treated casting is cut and otherwise machined into a camshaft A as a final product with desired dimensional accuracy.
It is the general practice to melt the raw materials in the atmosphere. When the raw materials are melted, therefore, nitrogen present in the atmosphere is dissolved into the molten metal and exists as free nitrogen in the molten metal. Consequently, defective products that are formed from the molten metal necessarily contains nitrogen. If such defective products are used as return scrap, then the contained nitrogen is accumulated in successive generations of return scrap, with the result that castings containing a large amount of nitrogen will be produced.
The region which contains the fine pearlite is very hard. Even if the chilled structure is eliminated, the structure which is highly resistant to wear remains in the casting, making it difficult to cut and otherwise machine the casting. It takes a long period of time to cut and otherwise machine the casting to desired dimensions, thus reducing the efficiency with which to produce the camshaft A.
The very hard casting is liable to crack or break when it is subjected to various machining processes. Stated otherwise, the yield of camshafts A produced from the very hard casting is inevitably low.
The highly hard casting is liable to crack or break when it is subjected to various machining processes. Stated otherwise, the yield of camshafts A produced from the highly hard casting is inevitably low.
While the above problems can be solved by suppressing the generation of fine pearlite in castings, no process has been available in the art for suppressing the generation of fine pearlite in castings.